Coordination chemistry and organometallic chemistry have intensely fuelled the field of transition metal catalysis, and knowledge-based ligand design is, next to combinatorial and high-throughput experimentation, a leading approach to develop new catalytic systems. For the industrially important hydroformylation reaction, ligand effects have been studied in detail, also with the aid of high-pressure spectroscopy techniques, (for example, HP NMR and HP-IR).[1] The active species in this reaction is a trigonal bipyramidal complex, with a hydride on the axial position. The classical Wilkinsons dissociative mechanism, based on triphenyl phosphine as ligand, is widely accepted and explains most observations made to date. Two coordination complexes have been observed with either the ligands coordinated in the equatorial-equatorial (eq-eq) or in the equatorial-axial (eq-ax) mode (Scheme 1).Van Leeuwen et al. elegantly demonstrated that bidentate phosphorus ligands with wide bite angles predominantly lead to the formation of the eq-eq rhodium complex, resulting in highly selective rhodium-catalyzed hydroformylation of 1-octene, and promoting the formation of the linear aldehyde product.[2] In the field of asymmetric hydroformylation, the most promising class of ligands are hybrid phosphines and phosphites or phosphoroamidites.[3] A real breakthrough in this field was achieved by Takaya, Nozaki et al. with the discovery of Binaphos, which gives ee values of up to 95 % for a wide range of substrates.[4] The reason for the exceptionally high enantioselectivity is attributed to the exclusive formation of a single active species, in which the ligand coordinates in eq-ax mode to the transition-metal center. Monodentate ligands have been much less applied in these transformations, as lower selectivities are anticipated owing to lower control over coordination modes. However, because of their simple structure, their synthesis is generally much less elaborate, making these ligands potentially cheaper and enabling the preparation of large ligand libraries, which are required to rapidly find new active and selective catalysts by rapid screening technologies. Application of very bulky monodentate phosphite (p-accepting) ligands demonstrated that these form very active catalysts that are also active in the hydroformylation of internal alkenes, albeit with significant isomerization.[5] Spectroscopy experiments on such systems indicate that only one phosphite ligand coordinates to the metal center in the equatorial plane. Along the same lines, Breit et al. have reported the use of bulky phosphabenezenes in hydroformylation catalysis.[6] In situ high-pressure NMR investigations demonstrate the prevalent formation of a monophosphine rhodium complex in which the ligand is located in equatorial position whereas the hydride occupies the axial site, which accounts for the reported activity and selectivity.Recently, we have introduced a ligand-template approach for the supramolecular encapsulation of transition-metal complexes.[7] Ligand-temp...